1. Field of the Invention
The present invention relates to an image processing apparatus such as an encoder or decoder for a color image used on a color facsimile terminal equipment which communicates the color image or a color electronic file for storing color images.
2. Related Background Art
Conventionally, various proposals have been made for the still color image encode method. The still color images of interest can be largely classified into the following two types.
(1) Binary color image in which eight colors of red, green, blue, cyan, magenta, yellow, white, black are dealt with in the binary form.
(2) Multi-level color image in which the representation comprising, for example, 16,700,000 colors is made by grading red, green and blue each into 256 gradients.
One of the binary color image encode methods is such that the modified Huffman method, which is a variable-length encode method currently used for the facsimile terminal equipment or the like, is applied to each color. In addition, the prediction encode method in which an encode pixel is predicted from surrounding pixels has been proposed. These methods fall into a category of the reversible encoding in which data preservation is made in the encode and decode.
On the other hand, as to the multi-level color image encode method, a method has been proposed in which a 8-bit signal for each RGB is converted into a luminance signal and a color difference signal, the coefficient values to which the orthogonal conversion (discrete cosine conversion) is applied are linearly quantized, and those quantized values are encoded in variable-length. This method is performed basically in such a way that image data is reduced by leaving the low-frequency portion of spatial frequency in image and cutting away the high-frequency portion. This method is a non-reversible encode method, with a trade-off relation between the compression ratio and the degradation of image quality.
However, in the conventional example as above described, when a color document image having color photograph incorporated therein is encoded, there are problems such as
(1) In the binary color method, even if the color document portion can be encoded with an increased compressibility, an image with gradients in the color photograph portion can not be efficiently encoded.
(2) In the multi-level color method, since with the compressibility being increased, the high-frequency components of image are mostly cut off, so that the quality of image at edge portions such as letters or the like is degraded, and thereby such a high compressibility of document as in the photograph portion can not be expected.
Thus, in the still color image transmitter, two types of encoders must be provided in order to encode an image with a high compressibility and a less amount of degradation.
The encode method of using multiple types of encoders has been proposed in U.S. patent application Ser. No. 631,145 and the new U.S. Patent Application filed on Feb. 5, 1991 on the basis of Japanese Patent Application Nos. 2-27658, 2-147035, 2-147036, 2-262798 and 2-262799.
On the other hand, in performing two types of encodes, encode areas may be overlapped.
For example, when a color photograph image (multi-level color image) exists in a color document image, as shown in FIG. 18, a problem arises if a multi-level color image area is encoded after the whole area is encoded with a binary color image encoder, because binary codes are redundant for the multi-level area.